A reliable non-destructive plastic deformation monitoring method is crucial for assuring safety and reliability of structural materials and mechanical structures. Plastic deformation can sometimes be determined by measuring dimension changes, but such methods are not an effective monitoring method for many practical applications due to production tolerance in size, localization of the plastic strain, and complex shapes of the plastic to be monitored.
Plastic deformation in an area produces high order harmonics in the acoustic signal propagated. Thus, high order harmonic acoustic signals as a nonlinearity parameter change to determine an extent of plastic deformation of a structural material has been proposed. Increase in the acoustic nonlinearity parameter is due to nonlinear stress-strain relation in the material produced by plasticity-induced microstructure changes.
Methods of nonlinear acoustic testing with discrete acoustic transducers has been proposed for studying plastic deformation and, while the correlation of nonlinearity parameter with the plastic deformation has been observed, there is a lack of an effective method to monitor plastic deformation in practical applications.
High order nonlinear acoustic signals are typically very small; they also exist in structural material without plastic deformation. Conventional bulky discrete acoustic transducers are fixed and positioned manually to measure the high order non-linear acoustic signals on the structural material to be monitored. Any variation in the pressure to fix the transducers or in the positioning or alignment of the discrete transducers significantly affects the measurements of the small harmonic signals.
In addition, for measuring Rayleigh or Lamb acoustic waves, bulky discrete acoustic transducers need be used together with wedges to convert the external transducers' vibration to a desired acoustic wave in the structural material. Conventionally, the wedges are manually fixed on the structural material, giving arise to many problematic factors having significant effect on monitoring of the plastic deformation such as positioning, alignment and type of the wedge. Moreover, acoustic coupling gel agent is typically required to be applied between the discrete transducer (or the wedge) and the structural material to facilitate transmitting acoustic waves. The amount and homogeneity of the coupling agent will also affect the testing results as even an extremely thin air gap at the transducer/material interface has significant detrimental effect on acoustic energy transmission. It is also challenging to use discrete transducers with structural material having curved shapes or limited space.
These issues with conventional plastic deformation monitoring method make it difficult to obtain repeatable or comparable high order harmonic acoustic signal reception, further acerbated by the fact that such high order harmonic acoustic signals are typically a few orders of magnitude smaller than fundamental signals.
Thus, what is needed are methods and systems for monitoring plastic deformation that are based on high order harmonic, such as second order signal variations, which provide reliable and consistent results despite service time or stress loading. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.